Vertically emitting microdisklasers

Lukas MahlerNEST CNR-INFM and Scuola Normale Superiore,

Pisa

Presented at the COMSOL Conference 2008 Hannover

People involvedAlessandro Tredicucci, Fabio Beltram

NEST CNR-INFM and Scuola Normale Superiore, Pisa

Christoph Walther, Maria Amanti, Jérôme FaistInstitute for Quantum Electronics, ETH Zürich

Bernd WitzigmannIntegrated Systems Laboratory, ETH Zürich

Harvey Beere, David RitchieCavendish Laboratory, University of Cambridge

The THz gap!

1µ

10µ

100µ

1m

10m

100m

1

10

100Electronics

1000100101.1.01

Photonics

Impatt

Gunn

III-V's

Lead salts

Output P

ower ( W

atts )

SLED

Frequency ( Terahertz )

Photo-mixer

RTD array

RTD

HGQC Laser

THz QCs!

Technology applications

Information Ultra fast signal processingMassive data transmissionWireless communications

Space ScienceCosmology Planetary, cometary Cosmochemistry

Environment Atmospheric sensing

MedicineImaging of biological tissue

Security Controls

Defense Chemical agent detectionDigital radar Imaging radar Covert communication Space-space

Short range battle field Transportation

Collision avoidance Material processing

Tomography

⇒Unknown applications created by new technology

The unipolar semiconductor laser

layer thickness

CB

material

CB

VB

diodelaser:

Quantum Cascadelaser:

“materials by design”: band structure engineering and molecular beam epitaxy (MBE)population inversion, matrix elements, scattering times, and transport are designed for optimum performance

1971: amplification from intersubband transitions is first postulated by R. F. Kazarinov and R. A. Suris Sov. Phys. Semicond. 5, 207 (Ioffe)

1994: QC-laser is first experimentally demonstrated by J. Faist et al. Science 264, 553 (Bell Labs)

2002: THz QC-lasers Nature 417, 156 (INFM Pisa-Cavendish Lab)

Distributed feedback resonator for THz QCL

• Double metal waveguide• Periodic slits in the top metallization

Very big coupling constant

Linear gratings

Radiative mode

M. Schubert et al. JQE 42, 257 (2006)

Non-radiative mode

Ey

Hx

zx

y

Device

0.0

0.2

0.4

0.6

0.8

1.0

3.0 3.1 3.2 3.30

20

40

60

80

100

120

Rad

iativ

e ef

ficie

ncy

Qua

lity

fact

or

Frequency (THz)

-30 -20 -10 0 10 20 300.0

0.2

0.4

0.6

0.8

1.0

Inte

nsity

(a.u

.)

Angle (deg)

Slab mode analysis

0.0 0.2 0.4 0.6 0.8 1.0Intensity (a.u.)

neff = 3.60-0.011i

Vertically emitting disks

• Full 3D simulation• Gold layers approximated with perfect electric

conductors

• Coupling mechanism for whispering galleries

• “infinite” grating

Simulation

0.0

0.2

0.4

0.6

0.8

1.0

3.00 3.05 3.10 3.15 3.20 3.25 3.300

20

40

60

80

100

120

140

Rad

iativ

e ef

ficie

ncy

Qua

lity

fact

or

Frequency (THz)

• Good quality factor• Reasonable outcouplingefficiency

Fabrication

• Etch the top contact layer in the slit region• Dry etched mesa

Measurement

102 104 106 108 110 112 114

170 μm

174 μm

178 μm

Inte

nsity

(a.u

.)

Frequency (1/cm)

182 μm

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.200

2

4

6

8

10

12

14

Inte

nsity

(a.u

.)

Current (A)

• Mode shifts perfectly

• High slope efficiency

• Pumping ok?

Which modes are lasing?

0.0

0.2

0.4

0.6

0.8

1.0

3.05 3.10 3.15 3.20 3.25 3.300

20

40

60

80

100

120

140

Rad

iativ

e ef

ficie

ncy

Qua

lity

fact

or

Frequency (THz)

• Try prime symmetry

• Pump only the circumference

Prime symmetry

3.05 3.10 3.15 3.20 3.25 3.30 3.350

20

40

60

80

100

120

140

Inte

nsity

(a.u

.)

Qua

lity

Fact

orFrequency (THz)

• 17 periods on the circumference• Remove the top contact layer in the center

0 20 40 60 80 100 120 140 160 1800

50

100

150

200

250 17-period grating 16-period grating

Pow

er (μ

W)

Current (mA)

Light-Current

0 20 40 60 80 100 120 140 160 180 2000

10

20

30

40

50

60

70

80

90

Pow

er (μ

W)

Current (mA)

NearfieldMagnetic field

In the device

Above the device

Farfield

-40 -20 0 20 40

-40

-20

0

20

40

Ang

le(d

eg)

Angle(deg)

16-fold symmetryradial mode

17-fold symmetrywhispering gallery mode

Computed farfield of a whispering gallery mode

-90 -60 -30 0 30 60 90

Inte

nsity

(a.u

.)

Angle (deg)

From microdisk to milliring

The farfield is determined by λ/d

-20 -10 0 10 20

-20

-10

0

10

20

Ang

le (d

eg)

Angle (deg)

-20 -10 0 10 20

-20

-10

0

10

20

Ang

le(d

eg)

Angle(deg)

1 mm diameter1.5 THz

Conclusions• Accurate prediction of spectral

emission properties is possible• A good understanding of the

coupling mechanism and spatial emission properties is obtained

Spherical farfield plot of a ring